Intense exercise bouts cause eccentric loading of cardiac muscle, leading to thickening and strengthening that increases stroke volume.

“The amount of blood being returned to the heart actually causes an eccentric loading of one of the muscular walls of the heart. Your heart is muscle, it's cardiac muscle. ... The left ventricle essentially getting slammed back and then having to push back, and a kind of eccentric loading of the cardiac muscle, and the muscle thickens. As more blood is returned to the heart, there's an adaptation where the heart muscle actually gets stronger and therefore can pump more blood per stroke, per beat.”